This invention pertains to the art of pumps and more particularly to electrically driven pumps capable for use as heart or blood pumps, or ventricular assist devices.
The invention is applicable to a pump for the pumping of blood of a living person, or animal, to replace or assist the pumping function of the biologic heart. It is capable of being sized and designed to fit inside a pumping chamber of a human heart, e.g. dimensioned on the order of a few inches. However, certain aspects of the invention could be readily configured for use in other environments than a blood pump wherein the presence of a shaft seal or long fluid residence times in the pump would be detrimental to the application.
In the preferred environment, conventional continuous flow rotodynamic blood pumps, which includes centrifugal blood pumps, axial flow blood pumps, or nonpulsatile blood pumps, have suffered from a number of problems. One major difficulty has been the presence of a shaft seal. Loss of blood from the circulatory system, and/or introduction of material of damaging types or quantities into the circulatory system must be prevented. Because of the intended use as life support for a human being, long life and utter reliability are key design goals. Using a contact type seal or fluid purged seal as the shaft seal results in significant difficulties meeting these objectives.
Most commercially available blood pumps, as exemplified by U.S. Pat. No. 4,135,253 to Reich, et al., U.S. Pat. No. 4,625,712 to Wampler, U.S. Pat. No. 3,647,324 to Rafferty, et al., or U.S. Pat. No. 4,589,822 to Clausen et al., have a blood pumping impeller mounted on a shaft which penetrates a side wall of the pumping cavity by means of a shaft seal. On the non-blood side of the seal, or drive compartment, is a motor or drive magnet, operating in air or a biocompatible fluid, which rotates the impeller via the shaft. A very common cause of failure of such prior known blood pumps is the eventual leakage of the seal about the shaft which either permits blood to enter into the drive compartment, excessive fluid leakage from the non-blood side into the blood, or both.
Other blood pump structures have been proposed to overcome the problems of blood pump shaft seals. Moise has a non-contacting housing-shaft interface with micron sized clearances that unfortunately introduce significant problems. Attempting to maintain these close dimensional tolerances clearly becomes a manufacturing problem. Additionally, there is a risk of blood leakage if the supply of purge fluid to the non-blood seal of the pump is interrupted.
Dorman U.S. Pat. No. 4,927,407 suggests using a constant flow pump such as a dynamic perfusion pump to supply a continuous, controlled flow of purge fluid to the seal. The purge fluid flow maintains the seal out of contact with the shaft. Unfortunately, this adds significant complication to the system because of the need for another active control pump and, even then, still requires a purge fluid for normal operation.
Alternatively, it has been suggested to magnetically suspend the impeller in three dimensions. This eliminates the need to breach the pump walls with a shaft to the impeller. Bramm, et al. and Moise generally illustrate this approach. However, at best this is a complex system because of its three dimensional nature, and is not presently believed to be commercially feasible.
Still other blood pumps have eliminated a penetrating shaft and accompanying seal by magnetically coupling the pump to a prime mover through a wall of the housing. Dorman U.S. Pat. No. 3,608,088 is an example of one such arrangement from the blood pump field. Although these eliminate leakage, they may amplify other problems also present in shaft sealed designs.
For example, high shear stresses around the seal, or around radial and axial bearings, can damage blood if the loads and shear stresses are not very low. Axial magnetic coupling, such as exemplified by Domnan, particularly creates an axial load which must be reacted or countered. Both shear and mechanical friction forces can produce heat that injures susceptible media. With blood, heat produced from the shear and friction forces can also promote deposition, including protein buildup, as well as a conventional clot.
Residence time is also a factor in handling some fluids such as blood. The amount of shear that blood can tolerate is a function of exposure time. Blood coagulation, leading to clots and emboli, also requires a finite time to occur. Thus many pump structures and principles derived from pumps in other fields are not suitable in the blood pump environment since they include relatively stagnant areas where the blood can settle out, react, decompose, or otherwise respond according to the environment and their sensitivity.
Some alternate pump design proposals employ a counter-impeller to prevent backflow along the shaft. This is particularly well known in other pump environments, i.e., non-blood pumps where the dynamic action of a counter-impeller provides a back flow along the shaft. These arrangements hold pumped fluid for long periods in the expeller, thus exposing the pumped fluid to shear. Therefore, these structures are not feasible for use as blood pumps because of all the problems described above associated with long residence times and shear. Further, these arrangements could accelerate leakage into the system if the second impeller discharges its prime of pumped fluid. Even further, these pump arrangements still require additional sealing in order to prevent fluid loss at static or low speed conditions.
The present invention contemplates a new and improved device which overcomes all of the above discussed problems and others to provide a new blood pump.